Merabet's brother Malek has spoken of how he now has to watch Ahmed "get slaughtered every day" after the video was used on international news channels.

"I had to speak to someone," Mir said of uploading the video. "I was alone in my flat. I put the video on Facebook. That was my error."

Mir said he left the video on Facebook for as little as 15 minutes before thinking the better of it and taking it down.

It was too late.

The footage had already been shared across the site and someone uploaded it to YouTube. Less than an hour after Mir removed the video from his page, he was startled to find it playing across his television screen.

In its unedited form, the 42-second film shows two masked gunmen - brothers Cherif and Said Kouachi - as they walk toward a prone police officer, later identified as 42-year-old Ahmed Merabet.

"You want to kill us?" one of the brothers says as he strides toward the wounded officer.

The video unleashed a worldwide wave of revulsion. British newspapers used stills from it on their front covers and described it as "shocking" and "sickening."

France's Le Figaro ran a still from the footage on its front page over a caption which read "War." CNN's Randi Kaye called it "an unforgettable image forever associated with this horrible attack."

The iconic nature of the imagery - rebroadcast again and again - has anguished Merabet's family. His brother Malek told journalists on Saturday: "How dare you take that video and broadcast it? I heard his voice. I recognised him. I saw him get slaughtered and I hear him get slaughtered every day."

Malek Merabet has spoken of the pain the video caused

Some argue that the video plays a useful role by exposing terrorists' heartlessness. Mir said that one official told him the video helped galvanise French public opinion.

"For me, the policeman killed, it's like a war photo," Mir said at one point, comparing it to famed photographer Robert Capa's controversial picture of a soldier being shot dead during the Spanish Civil War.

The video did help cause an outpouring of support for Merabet and his family, with many adopting the tag "Je Suis Ahmed" - I am Ahmed - as a spin on the solidarity slogan "Je Suis Charlie."

Article continues below video

As Mir spoke on Saturday, members of the public were still gathering at the site of Merabet's death to lay flowers and pay respect.

Mir didn't even know what he was filming at first. Drawn to his window when the sound of gunshots interrupted his emailing, he initially thought there was a bank robbery in progress. When he spotted the rifle-wielding men in black walking down the street, he assumed they were SWAT police going to help a stricken comrade.

"And - horror - they're not," Mir said.

As police rushed to the scene, Mir downloaded the video to his computer and then to a removable disk, which he handed to officers.

Then, he uploaded the footage to Facebook - and to the world.

Mir, a slight man in his 50s whose parents were refugees from fascist Spain, is still at a loss to explain exactly what pushed him to share the chilling video with his 2,500 Facebook friends.

"There's no answer," he said. Perhaps a decade of social networking had trained him to share whatever he saw.

"I take a photo - a cat - and I put it on Facebook. It was the same stupid reflex," he said.

Mir wanted Merabet's family to know he was "very sorry," saying that he had turned down offers to buy the footage and that he wanted media organisations to blur Merabet's image before running it. But many, he said, just broadcast the unedited footage without permission.

Mir said that, if he could do it all again, he would have kept the video off Facebook.

"On Facebook, there's no confidentiality," he said. "It's a lesson for me."